Add completion spec: pkill (pkill)

[maro20066600]

Summary

• add a pkill command signature with process-name completions
• add dynamic generators for process names and user-name arguments
• register the pkill generators in command-signatures

Related to warpdotdev/warp#10974 and warpdotdev/warp#10924.

Testing

• npm run format -- command-signatures/json/pkill.json
• cargo fmt --check
• cargo test -p warp-command-signatures pkill
• cargo test -p warp-command-signatures
• script/presubmit

[lucieleblanc]

Hi @maro20066600 , thank you for the contribution. Since this PR adds generators, could you provide screenshots of the generators working in a local build of Warp? This process is explained in the test-local-warp skill.

I'll be happy to review this again once screenshots are uploaded and displayed in the PR description or a comment. Thanks!